Recently, printing technology has been remarkably improved through the introduction of offset printing, color printing, high-speed, large-scale printing, automation, and the like. With such progress, improvement in various physical properties of printing paper is demanded from the viewpoints of workability and printing adaptability.
Newsprint is mainly composed of mechanical pulp (referred to as MP hereinafter) and deinked pulp (referred to as DIP hereinafter) and classified into medium-grade paper or low-grade paper. However, the quality requirements for newsprint are stricter than those for general printing paper because in newspaper printing a specified large number of newspapers have to be securely printed within a specified duration of time in a specified time zone. Since a reduction in weight and an increase in the DIP ratio are required for current newsprint, improvement in various respects has to be made considering these and other requirements. Thus, the improvement of newsprint requires a much higher level of technology as compared to that of general printing paper.
As regards to the newspaper printing system, transition to offset printing has proceeded rapidly together with the introduction of computer systems so that today the offset printing system is predominantly used. Further, as offset color printers such as satellite-type and tower-press-type printers have come into wide use, color printing space in newspaper increasingly tends to be expanded. Due to the use of currently popularized four color offset printing, in which the amount of adhering dumping water increases about four times and the transfer of the dumping water causes swelling of newsprint, a pitch problem (dot gaps) is generated, resulting in possible color registration errors with poor image resolution on the printed surface.
Due to the increased popularity of offset printing, quality requirements for newsprint today are different from those for relief printing newsprint; for example the following qualities are required:
(1) paper should have wet strength and not suffer from water break;
(2) paper should retain adequate water absorption resistance;
(3) paper should have a small peeling strength (Neppari); and
(4) paper powder should not be generated.
Among these quality requirements, provision of water absorption resistance (in other words, provision of sizing property) is an important objective.
Further, in addition to the use of offset printing system, a lesser use of GP due to an increase in DIP content, popularization of neutral papermaking, reduction in the basis weight of newsprint and the like are also considered as factors tending to cause the swelling of fibers and generate color registration errors.
In response to the quality requirements for newsprint paper, such as a higher whiteness level and improved clearness in color printing, so-called neutral papermaking has recently become a major trend, in which base paper for newsprint is manufactured at a pH in the range from neutral to weak alkaline. Due to this transition to neutral papermaking, the addition ratio of aluminum sulfate in newsprint production is reduced so that the effect (provision of water absorption resistance) of a surface sizing agent conventionally used in newsprint base paper produced in acidic papermaking (hereinafter referred to as acidic newsprint base paper) tends to be reduced. The surface sizing agent used for acidic newsprint paper is generally a copolymer of a monomer containing a carboxyl group and a styrene monomer. It is presumed that the interaction between the carboxyl group in the molecule of this surface sizing agent and an aluminum component in base paper for newsprint aligns the molecule of the surface sizing agent so that the hydrophobic monomer part having the carboxyl group is positioned inside the paper and the hydrophobic styrene monomer part is positioned on the surface of the paper, thereby providing water absorption resistance to the newsprint. However, also in base paper for newsprint having a low aluminum sulfate addition ratio, particularly in base paper for newsprint produced by a neutral papermaking process (hereinafter referred to as neutral newsprint base paper), the abovementioned alignment of the surface sizing agent is not as complete as in acidic newsprint base paper, which results in a huge decrease in water absorption resistance when compared to the case where the same amount of the abovementioned surface sizing agent is used for coating.
As mentioned above, it has been difficult to provide water absorption resistance to neutral newsprint at the same level as to acidic newsprint using surface sizing agents conventionally used for acidic newsprint. Further, in acidic newsprint, further improvement of water absorption resistance is desired.
Water absorption resistance has been conventionally controlled in the same manner as in general printing paper by adding sizing agents inside the paper (internal addition sizing) or by adding the agents outside the paper (external addition sizing). The internal addition is a means of adding an agent to pulp slurry at a so-called wet-end and make the agent to be contained in the inside of paper simultaneously at the time of papermaking. The external addition is a means of coating an agent onto the surface of base paper using a coating machine such as a two-roll size press and a gate roll coater after papermaking.
Examples of generally used sizing agents for internal addition include fortified rosin sizing agents, emulsion-type sizing agents, and synthetic sizing agents for acidic papermaking; and alkyl ketene dimers (AKDs) and alkenyl succinic anhydrides (ASAs) for neutral papermaking. Various problems are caused by using internal addition of such sizing agents, as follows:
(1) an agent should be added to pulp slurry at a low concentration;
(2) the amount of an agent fixed on pulp sheet is not constant (the amount of the agent fixed is low);
(3) manufacturing of paper that does not require water absorption resistance cannot be performed simultaneously when multiple numbers of papermaking machines are sharing the same circulatory white water;
(4) the effect of a retention aid is unstable; when the retention is increased, colored foreign materials derived from DIP are also integrated into the sheet;
(5) water absorption resistance changes with time;
(6) the retention of an internal addition sizing agent tends to decrease when a high-speed papermaking machine is used for manufacturing neutral and/or light-weighted newsprint, which makes it difficult to provide water absorption resistance, and
(7) the retention of an internal addition sizing agent tends to decrease when newsprint containing DIP at 80% or more is manufactured using a high-speed papermaking machine at 1,000 m or more/minute; this makes it difficult to provide water absorption resistance.
Accordingly, when a means of internal addition of a sizing agent is applied, the control of the amount of the sizing agent to add is difficult and the amounts of the internal addition sizing agent and retention aid have to be increased or decreased depending on the circumstances. The internal addition sizing agent is added in an excessive amount when its effect is insufficient, which tends to cause a decrease in paper strength, generation of marked stains in white water system caused by adhesion and accumulation of a hydrophobic sizing agent and the like, resulting in problems in cost, quality and operation conditions.
A papermaking machine for manufacturing newsprint generally is equipped with a gate roll coater for coating an agent onto the surface of newsprint base paper. In the abovementioned conventional technology for providing water absorption resistance to newsprint, a styrene surface sizing agent, which is a copolymer of a monomer containing a carboxyl group and a styrene monomer, is generally used for acidic newsprint as mentioned above. However, sufficient water absorption resistance cannot be attained when this styrene surface sizing agent is used for coating neutral newsprint added with a small ratio of aluminum sulfate, in particular neutral base newsprint.
The present inventors have been continuously studying on the provision of water absorption resistance to newsprint by external addition of sizing agents and have already made patent applications as follows. There is described a method of forming a coating layer containing a composition to control water absorptivity mainly composed of component A, component B and component C on base paper for printing (particularly newsprint). Component A: modified starch or starch; component B: at least one polyacrylamide selected from nonionic polyacrylamides, cationic polyacrylamides having a tertiary amine group, cationic polyacrylamides having a quaternary ammonium group, and amphoteric polyacrylamides; and component C: an anionic copolymer of a monomer having a weight average molecular weight of 1,000 to 3,000,000 and a hydrophobic substituent having 6 to 10 carbon atoms and a monomer having a carboxyl group or a sulfonic acid group (see Japanese Patent No. 2939971). There is described a method of manufacturing a neutral newsprint, in which a ketene dimer sizing agent and a paper surface treating agent are externally added using a gate role coater to neutral newsprint in which calcium carbonate is used as a filler, after which the coated paper is passed through a soft calender at a surface temperature of 50° C. or higher to attain the sizing degree (see Japanese Patent No. 2980833). There is described a method in which a coating layer containing an absorptivity controlling composition mainly comprised of two components, component A and component B, is formed on base paper for printing (particularly newsprint base paper) to achieve a droplet water absorption degree of 10 to 1,000 seconds. Component A: at least one polyacrylamide selected from nonionic polyacrylamides, cationic polyacrylamides, and amphoteric polyacrylamides; and component B: an anionic copolymer of a monomer having a hydrophobic substituent and a monomer having a carboxyl group and/or a sulfonic acid group (see Japanese Patent No. 3093965). There is described a method in which a coating layer containing a surface sizing agent mainly comprising three components consisting of the following component A, component B, and component C, or two components mainly comprising component B and component C at a ratio by solid weight of each component of A:B:C=0-80:95-20:1-10 is formed on newsprint base paper. Component A: at least one polyacrylamide selected from nonionic polyacrylamides, cationic polyacrylamides, and amphoteric polyacrylamides; component B: an anionic ammonium salt of a copolymer of a monomer having a hydrophobic substituent and a monomer having a carboxyl group, and component C: at least one resin acid selected from dehydroabietic acid, abietic acid, dihydroabietic acid, pimaric acid, neopimaric acid, isopimaric acid, levopimaric acid, and palustrine, or rosin containing these resin acids (see Japanese Patent No. 3303291).
Surface sizing agents shown in the abovementioned conventional methods and sizing agents to be used in the present invention can be common in terms of providing water absorption resistance; however, a surface sizing agent to be used in the present invention is novel and has a different composition.
Further, in order to prevent accumulation of paper powder on a blanket and the resulting faint printing problem in offset printing in which printing ink having a relatively strong tackiness is used, there are prior technologies with an objective to increase the surface strength and water resistance of newsprint, as described below.
There are disclosed newsprint for offset printing in which base paper is coated with an aqueous solution containing a surface treating agent and dried, characterized in that said surface treating agent comprises at least a polyacrylamide polymer and an epoxy water-resistant agent and/or a polyvalent metal compound water-resistant agent (see Japanese Patent Application Laid-open No. Hei 10-259592); newsprint for offset printing in which base paper containing an internally added filler is coated with a surface treating agent, characterized in that said surface treating agent contains a polyvinyl alcohol copolymer having a silanol group (see Japanese Patent Application Laid-open No. Hei 11-21790); newsprint for offset printing in which base paper is coated with a surface treating agent and dried, characterized in that said surface sizing agent mainly comprises a synthetic resin latex having a gel content of 90% or more by weight (see patent Japanese Patent Application Laid-open No. Hei 11-50393); newsprint for offset printing in which base paper is coated with a surface treating agent and dried, characterized in that said surface treating agent mainly comprises a copolymer latex and contains a release agent (see Japanese Patent Application Laid-open No. Hei 11-158795); newsprint for offset printing in which both sides of base paper are coated with a surface treating agent and dried, characterized in that said surface treating agent mainly comprises an acrylic alkali swellable synthetic resin latex (see Japanese Patent Application Laid-open No. 2000-17597); newsprint for offset printing in which both sides of base paper are coated with a surface treating agent and dried, characterized in that said surface treating agent mainly comprises (a) starch or modified starch and (b) a hydrophobic acrylic surface sizing agent containing butyl (meth)acrylate and/or (meth)acryl 2-ethylhexyl as monomer components and having a glass transition temperature of 10° C. or lower, the ratio of said starch component to said hydrophobic acrylic surface sizing agent being in the range of 100:3 to 100:30 by solid weight (see Japanese Patent Application Laid-open No. 2002-294588).
[Patent reference 1] Japanese Patent No. 2939971
[Patent reference 2] Japanese Patent No. 2980833
[Patent reference 3] Japanese Patent No. 3093965
[Patent reference 4] Japanese Patent No. 3303291
[Patent reference 5] Japanese Patent Application Laid-open No. Hei 10-259591
[Patent reference 6] Japanese Patent Application Laid-open No. Hei 11-21790
[Patent reference 7] Japanese Patent Application Laid-open No. Hei 11-50393
[Patent reference 8] Japanese Patent Application Laid-open No. Hei 11-158795
[Patent reference 9] Japanese Patent Application Laid-open No. 2000-17597
[Patent reference 10] Japanese Patent Application Laid-open No. 2002-294588